1. Field of the Invention
The present invention relates to a vertical heat-treating apparatus used in the thermal diffusion step or film-forming step in the process of manufacturing a semiconductor device.
2. Description of the Related Art
Researches on a vertical heat-treating apparatus used in the thermal diffusion step or film-forming step in the process of manufacturing a semiconductor device are being made vigorously in recent years in an effort to save the required space and required energy. The vertical heat-treating apparatus is also effective for dealing with the tendency toward enlargement in diameter of the semiconductor wafer and with tendency toward automation of the process for manufacturing a semiconductor device.
The conventional vertical heat-treating apparatus comprises a vertical heat-treating furnace consisting of a cylindrical reaction vessel made of, for example, quartz and a heater, a heat insulating material, etc. arranged about the outer surface of the reaction vessel. A wafer boat made of, for example, quartz and having a number of semiconductor wafers housed therein is loaded into a lower part of the vertical heat-treating furnace and moved upward by a lift mechanism to reach a predetermined position for applying a predetermined heat treatment to the wafers. In general, semiconductor wafers are put in a wafer carrier made of resin for transporting the wafers, making it necessary to transfer the wafers between the wafer boat and the wafer carrier. Thus, the vertical heat-treating apparatus is provided in general with a wafer transfer mechanism positioned between the wafer boat and the wafer carrier.
It is desirable to use a plurality of wafer boats such that, during the heat treatment applied to semiconductor wafers held by a wafer boat within a heat-treating furnace, other wafers are transferred onto another wafer boat and the wafer boat supporting said wafers is put in a waiting position so as to suppress the reduction in the heat-treating efficiency cause by the wafer transfer time between the wafer boat and the wafer carrier. It is also desirable to provide a section in which the wafer boat supporting the wafers is disposed in preparation for the heat treatment of the wafers and to arrange a plurality of vertical heat-treating furnaces in parallel so as to permit a single wafer transfer mechanism to be commonly used for the plural heat-treating furnaces and, thus, to improve the heat-treating efficiency.
As described above, it is desirable to put a wafer boat having semiconductor wafers mounted thereon in a waiting position so as to improve the treating efficiency. It should be noted, however, that an increase in the waiting time of the semiconductor wafer leads to an increase in the formation of a natural oxide film on the wafer surface. Presently, the semiconductor element is made finer and finer with increase in the integration density, with the result that a defective wiring is brought about by the presence of a natural oxide film as thin as only several nanometers. In addition, the natural oxide film is nonuniform in thickness, with the result that the defective wiring is likely to be brought about more easily. Naturally, it is of high importance to eliminate the natural oxide film on a semiconductor wafer.
As described above, it is very important to improve the heat-treating efficiency and to suppress formation of a natural oxide film on a semiconductor wafer. It is also important to eliminate the natural oxide film formed on a wafer before the wafer is subjected to a heat treatment.
What should also be noted is that a heat-treating apparatus used in the manufacture of a semiconductor device is disposed in general in a clean room. Naturally, a clean air is supplied to each equipment included in the heat-treating device in an attempt to prevent dust being attached to the wafer. However, it is impossible to completely prevent the dust attachment because of convection of the air. It follows that an increase in the waiting time of the semiconductor wafer enhances the possibility of the dust attachment to the wafer. Naturally, it is very important to take measures for preventing the dust attachment to the wafer during the waiting time of the wafer.